The present invention relates to a highly sensitive electron tube for quantitatively measuring an extremely weak light.
For measuring an extremely weak light, conventionally known is an electron tube as a light detector in which a charge coupled device (CCD) is used as an anode. Particularly in an electron tube described in Japanese Patent Publication No. Hei-7-95434, electron emitted from a photocathode in response to an incidence of light is directed into a rear side of a device formation surface to detect a signal. Such electron tube is widely used because of high sensitivity and high imaging quality.
An electron tube using a rear side irradiation type CCD generally includes a sleeve having two openings, an input plate provided with a photo-cathode adapted for emitting electron corresponding to the incident light, and a stem provided with the CCD. The input plate and the stem are adhered to the sleeve to plug the openings, and a vacuum is provided in an interior defined by the sleeve, the input plate and the stem. Further, the CCD is fixed to the stem in such a manner that the rear surface of the CCD is in confrontation with the input plate, and the entire CCD is positioned in the interior of the electron tube. At a front side of the rear irradiation type CCD, there are provided, on a semiconductor substrate, a SiO2 layer, charge storage electrode layer and a charge transmission electrode layer. The charge incident on the rear surface of the CCD is capable of being accumulated and transferred.
Here, since the photocathode can be formed only in a vacuum condition, in a process for making the electron tube, alkali metal such as Na, K and Cs is introduced into an interior of the electron tube in a vacuum condition, and the metal is acted on the input plate to form the photocathode. However if the alkali metal may be entered into the charge transmission portion on the substrate of the CCD which is a semiconductor element, and if the alkali metal reaches a gate SiO2 film, fixed charge and interface state may be increased, and therefore, the CCD characteristic is extremely degraded.
In order to eliminate the above-described problem, the present inventors have investigated the following methods. That is, a glass is bonded to the surface of the CCD by anodic bonding to protect the CCD against the alkali metal. However, due to the significant difference in thermal expansion coefficient between the glass and silicon which is the basic material of the CCD, high stress is imparted on the CCD in a subsequent high temperature process, and as a result, the CCD is damaged.
In an alternative method, the surface of the CCD is covered with a resin to protect the CCD against the alkali metal. However, the resin cannot withstand the subsequent high temperature process, and a gas is generated from the resin to degrade vacuum condition in an interior of the electron tube.
Further, a vacuum is normally provided in an inside of the electron tube, and insulation material in the electron tube is easily chargeable because of the application of high negative voltage to the photocathode for operation. Therefore, highly insulative SiO2 layer at the surface of the CCD and the insulation material in the tube are extremely strongly charged. Here, the electron tube employing rear surface irradiation type CCD is adapted for detecting extremely weak signal such as a charge of about several tens of electrons, and therefore, the charging on the surface of CCD and on the ambient insulation material may become a large noise source to excessively lower the detection accuracy.
In view of the above-described problems, it is an object of the present invention to provide a highly sensitive electron tube capable of avoiding degradation of CCD characteristic due to the internally introduced alkali metal with preventing the CCD from being subjected to excessive thermal stress during manufacture of the electron tube, and avoiding gas discharge and capable of moderating charging on the surface of the semiconductor element and its ambient components.
These objects of the present invention will be attained by an electron tube including a vacuum container, a cathode sealed in the vacuum container and having a photocathode surface containing an alkali metal, and an anode sealed in the vacuum container and having a rear side irradiation type semiconductor device formed with a charge transmission portion, the anode having a rear surface side in confrontation with the photocathode surface. The rear side irradiation type semiconductor device includes a flattened film, an electrically conductive lead, and a thin film. The flattened film covers the charge transmission portion and has a flattened top surface. The lead is formed on the flattened film and is electrically connected to the charge transmission portion. The thin film is formed over the flattened film and the lead, and is mainly composed of a silicon nitride.
As a result of the formation of the device, the front surface side of the rear side irradiation type semiconductor device becomes irregular, which involves high stress. By forming the flattened film over the device forming portion such as the charge transmission portion at the front surface side, the front surface becomes flattened, to protect the semiconductor device from application of excessive stress. Further, by the flattened surface at the front surface side of the semiconductor device, an electrical lead and a thin film mainly composed of silicon nitride can be easily formed.
Further, the thin film mainly composed of silicon nitride and formed over the flattened film and the lead can prevent alkali metal from being entered into the semiconductor device, the alkali metal being used for activating the photocathode surface. Thus, degradation of characteristic of the semiconductor device can be avoided. By the formation of the silicon nitride film over the uppermost surface of the semiconductor device serving as an anode, the alkali metal introduced into the tube cannot be entered into the semiconductor device. Consequently, high sensitivity can be obtained.
Furthermore, the flattened film formed below the silicon nitride film can avoid peeling of the silicon nitride film, and can moderate stress at the boundary therebetween.
Preferably, a SiO2 layer is formed over the surface of the charge transmission portion. The flattened film is made from a phosphosilicate glass and is formed between the SiO2 layer and the thin film.
By providing the flattened film made from phosphosilicate glass between the thin film mainly composed of silicon nitride and the SiO2 layer forming the surface of the semiconductor device, any stress due to the difference in thermal expansion coefficient among the conductive lead, the thin film mainly composed of silicon nitride and the SiO2 layer can be moderated. Further, by forming, over the topmost surface of the semiconductor device, the thin film mainly composed of silicon nitride having an electrical conductivity higher than SiO2, charging to the surface of the semiconductor device and its ambient portion can be moderated. As a result, any affect due to unwanted electrical charge imparted on the semiconductor device can be moderated, and highly sensitive device can be obtained.
Preferably, the charge transmission portion has a terminal portion, and a through hole is formed in the flattened film at a position above the terminal portion. The lead is electrically connected to the terminal portion through the through hole. With this arrangement, signal at the charge transmission portion can be easily transmitted to the lead.
Further, preferably, the lead has a bonding pad, and a through hole is formed in the thin film at a position above the bonding pad so as to expose the bonding pad, and an electrically conductive member is arranged in the through hole for connection to the bonding pad. With this arrangement, signal transmission between an external device and the lead can be easily performed.